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In a recent experiment it was shown that despite having an indirect band gap, the dominating peak in the emission spectrum of bilayer MoS₂ corresponds to direct transitions. [1] To understand this phenomenon, we have applied density-matrix based time-dependent density-functional theory to examine ultrafast charge dynamics and emissive properties of bilayer MoS₂. In particular, we demonstrate that despite initial accumulation of excited charge at the valleys between the K and Γ points of the two-dimensional Brillion zone, photoemission takes place through direct charge recombination at the K and K′ points. Analysis of the phonon spectrum suggests that the main reason for the direct emission is phonon-assisted transfer of the excited electrons to the K and K′ valleys. We also analyze the role of the spatial structure of the electron and hole excitations in the ultrafast charge dynamics and hence photoemission to find that d(Mo)-p(S) hybridized character of the holes facilitates inter-layer charge transfer. Our results thus reveal the importance of ultrafast charge dynamics in photoemissive properties of a few-layer transition-metal dichalcogenides.
[1] K. Xiao et al., private communication